It has been known for a long time that complex tones comprising only upper harmonics can evole a fundamental pitch sensation. More specifically, it has been shown that listeners can track a fundamental melody or melodic interval in a sequence of complex tones of randomly chosen upper harmonics. For an adequate understanding of music percepton, however, it is important to examine not only how melodic, but also how harmonic information is conveyed. Consequently, a series of harmonic interval identification experiments has been proposed in which notes are represened by two-tone harmonic complexes and two notes are presented simultaneously for identification of their (missing) fundamental frequency ration or harmonic interval. It is expected that subjects generally will be able to recognize harmonic intervals from simultaneous pairs of upper harmonics because this seems to happen all the time in live music. A positive outcome could, however, present a serious challenge for current pitch theories which are usually formulated in such a way that they extract only one pitch out of each simultaneous cluster of partials.